Revealing his Senate forecast on Sunday's "This Week" on ABC, Silver put the odds of Shaheen retaining the seat at 75 percent. The numbers were less favorable for Democrats in general, with FiveThirtyEight giving Republicans a 60 percent chance of taking control of the upper chamber.

The numbers aren't the only thing stacked against Scott Brown, however. History points to only two senators in the late 19th century who have represented multiple states in the history of the chamber, as Brown will attempt to do.